CHAPTER I. VARIATION UNDER DOMESTICATION.

3. CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BETWEEN VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM ONE OR MORE SPECIES.

When we look to the hereditary varieties or races of our domestic animals
and plants, and compare them with closely allied species, we generally
perceive in each domestic race, as already remarked, less uniformity of
character than in true species. Domestic races often have a somewhat
monstrous character; by which I mean, that, although differing from each
other and from other species of the same genus, in several trifling
respects, they often differ in an extreme degree in some one part, both
when compared one with another, and more especially when compared with the
species under nature to which they are nearest allied. With these
exceptions (and with that of the perfect fertility of varieties when
crossed--a subject hereafter to be discussed), domestic races of the same
species differ from each other in the same manner as do the closely allied
species of the same genus in a state of nature, but the differences in most
cases are less in degree. This must be admitted as true, for the domestic
races of many animals and plants have been ranked by some competent judges
as the descendants of aboriginally distinct species, and by other competent
judges as mere varieties. If any well marked distinction existed between a
domestic race and a species, this source of doubt would not so perpetually
recur. It has often been stated that domestic races do not differ from
each other in characters of generic value. It can be shown that this
statement is not correct; but naturalists differ much in determining what
characters are of generic value; all such valuations being at present
empirical. When it is explained how genera originate under nature, it will
be seen that we have no right to expect often to find a generic amount of
difference in our domesticated races.

In attempting to estimate the amount of structural difference between
allied domestic races, we are soon involved in doubt, from not knowing
whether they are descended from one or several parent species. This point,
if it could be cleared up, would be interesting; if, for instance, it could
be shown that the greyhound, bloodhound, terrier, spaniel and bull-dog,
which we all know propagate their kind truly, were the offspring of any
single species, then such facts would have great weight in making us doubt
about the immutability of the many closely allied natural species--for
instance, of the many foxes--inhabiting the different quarters of the
world. I do not believe, as we shall presently see, that the whole amount
of difference between the several breeds of the dog has been produced under
domestication; I believe that a small part of the difference is due to
their being descended from distinct species. In the case of strongly
marked races of some other domesticated species, there is presumptive or
even strong evidence that all are descended from a single wild stock.